After months of not been able to get rid of the pop up ads on youtube, the ones that take front position and block the video of what you're viewing I think I've finally got rid of them!
I'd previously deleted all my cookies in Firefox, but that didn't work, this time I deleted just the ones for youtube and then closed the browser, re-opened, went to youtube and then once again deleted all youtube cookies. This has worked for the past 10 videos I've watched, ads display correctly and videos play all the way through without problems.
To delete cookies:
"remove individual cookies"
search the cookies for youtube
Then hit Remove all cookies
This for me has stopped the random ads from popping up when I'm watching a video; they were really annoying especially if they had audio and it stops you seeing the Youtube video.